Why Tiny Fruit Flies Have Six Centimeter-Long Sperm

When it comes to males attracting mates in the animal kingdom, large ornamentation is where it’s at: peacocks with their tail feathers, elk with their antlers, and lizards with their dewlaps (the weird bulging, red lump in their throat). Fruit flies meanwhile remain completely unremarkable to look at; unless you take a peek at their sperm, that is.

At six centimeters long–over twenty times their body size–fruit fly sperm constitutes the most disproportionately large male ornament around, and until recently no one was entirely sure why. But thanks to the work of Scott Pitnick, an evolutionary biologist at Syracuse University who sports a full sleeve tattoo of a long-tailed sperm, we now have an idea.

Relative sizes of male ornaments in the animal world.

In a newly published study in Nature, Pitnik explains that the reason fruit flies’ sperm has excessively long tails is twofold. Firstly, it’s for the same reason males of any species tend to do anything flashy: because the females like it.

But unlike a peacock with very long tail feathers or a frat boy with a sports car, the female fruit fly isn’t conspicuously drawn to single mate with an outward trait. Rather, the female fruit fly has several mates, and her reproductive tract and large sperm storage organ (yes, really) has a bias towards longer sperm. And the longer the sperm, the more likely it is to graduate from sitting in a storage organ to fertilizing an egg.

To the right, the massive sperm storage organ of the female fruit fly.

The second reason is the desire of female fruit flies to produce the strongest offspring. When sperm are small and cheap to produce, then an animal can produce masses of them, but when you’re creating sperm that is twenty times ones body size, production is rather inevitably lower. So, as Pitnik explains:

As sperm evolve to be longer, only those males in the best health–those with good genes–can make large numbers of them and so capitalize on all the female remating. This too strengthens the female preference for longer sperm, as they get good genes to pass on to their offspring.

It’s all a bit baffling–not least in terms of the sheer mechanics of it. But as far as Pitnik is concerned, the male fruit fly’s massive sperm is perhaps the most logical of all male ornaments:

When you think about sex differences, people typically are thinking about gaudy males with dramatic courtship dances and females that are drab and demure. What they should be thinking of is sperm and eggs, because that represents the more fundamental difference between the sexes. When you’re thinking about outrageous ornaments that differentiate males and females, you should also be thinking about sperm first.